na-adjective
Describes a state of looking weak, powerless, or dispirited, often used for a person's appearance or manner.
彼は力無げにうなずいた。
He nodded feebly.
力無げな様子で座っている。
Sitting there looking helpless.
力ない is a more common adjective with a similar meaning of 'powerless' or 'feeble', while 力無げ is a less frequent na-adjective emphasizing the appearance of weakness.
Compound of 力 (chikara, 'power') + 無げ (nage, '-less seeming'), from 無い (nai, 'not') + げ (ge, suffix indicating appearance).